In cellular or radio communication networks, user equipments, UEs, such as for example mobile telephones or laptops communicate with radio base stations, RBSs, or nodes over a Radio Access Network. Depending on several factors, for example the distances between the different UEs and the RBS/node; or the number of UEs currently transmitting signals to or receiving signals from the RBS; or the geographical conditions between the different UEs and the RBS, the UEs transmit signals to the RBS with different transmission powers. The different transmission powers of the UEs cause several problems or issues which need to be considered. One issue is that the higher the power, the higher is the strain on the battery of the UEs causing the batteries of UEs being depleted more rapidly. A further issue, which is much more complicated, is interference. As a UE transmits with relatively high transmission power, the more interference the UE causes other UEs in its vicinity and also to neighbouring RBSs or nodes.
In order to address these issues, dynamic control of the transmission power of the UE or mobile station has been introduced. Dynamic control of mobile radio station transmission power (sometimes referred to as uplink power control) is thus a common feature in cellular systems. Some objectives of uplink power control include: (a) attaining a sufficient received power and signal quality on the used uplink radio channel at the serving RBS, (b) limiting the received power (interference) at non-serving RBSs, (c) limiting the received power (interference) on other channels at the serving RBS, and (d) reducing the output power level to limit power consumption and save battery life in the mobile station.
Power control schemes can further be divided in to the categories ‘closed-loop’ and ‘open-loop’ depending on what type of measurement input is used. Closed-loop schemes make use of measurements on the same link direction that the power control applies to, i.e., on the uplink for uplink closed loop power control. Open-loop schemes make use of measurements on the opposite link direction, i.e., on the downlink for uplink open-loop power control. Closed-loop schemes are typically more accurate than open-loop schemes, but also require more control signalling overhead.
Improved support for heterogeneous network operations is part of the ongoing specification of 3GPP LTE (Long Term Evolution) Release-10, and further improvements are discussed in the context of new features for Release-11. In heterogeneous networks, a mixture of cells of differently sized and overlapping coverage areas are deployed. One non-limiting example is illustrated below where pico cells or pico RBSs are deployed within the coverage area of a macro RBS or macro cell. A pico RBS is an example of a low power RBS having a coverage area commonly referred to as a low power cell or in this example, a pico cell. A regular RBS is in the context of heterogeneous networks referred to as a macro RBS. The macro RBS has a coverage area referred to as a macro cell. In FIG. 1, one macro RBS 100 is shown having a coverage area or cell 101. Within the cell 101, three different low power RBSs or pico RBSs 110, 120 and 130 are deployed. Each of the pico RBSs has a corresponding cell 11, 121 and 131 respectively.
Throughout this disclosure, nodes or points in a network are often referred to as being of a certain type, e.g., “macro” or “pico”. These types are only examples of such nodes or points and should not be interpreted as an absolute quantification of the role of the node/point but rather as a convenient way to illustrate the roles of different nodes/points relative each other. Thus, a description about macro and picos could, for example, just as well be applicable to an interaction between micros and femtos. Other non-limiting examples of low power nodes or points in heterogeneous networks are home base stations and relays. A large difference in output power (e.g. 46 dBm in macro cells and 30 dBm or less in pico cells) results in different interference situations as compared to networks where all base stations have the same output power.
Deploying low power nodes such as e.g. pico nodes or pico RBSs within a macro coverage area improves system capacity by cell splitting gains and also provides users, i.e. users of mobile stations or UEs, with a wide area experience of very high speed data access throughout the network. Heterogeneous deployments also cover traffic hotspots well. Hotspots are small geographical areas with high user densities served by, e.g., pico cells, and they represent an alternative deployment to denser macro networks.
A basic way to operate a heterogeneous network is to apply frequency separation between the different layers, i.e., the different macro and pico cells/RBSs operate on different non-overlapping carrier frequencies, and thereby avoid any interference between the layers. With no macro cell interference towards the under-laid cells, cell splitting gains are achieved when all resources can simultaneously be used by the under-laid cells. A drawback of operating layers on different carrier frequencies is that it may lead to resource-utilization inefficiency. For example, if there is low activity in the pico cells, it could be more efficient to use all carrier frequencies in the macro cell and then basically switch off the pico base stations. Nevertheless, the split of carrier frequencies across layers is typically done in a static manner.
Another way to operate a heterogeneous network is to share radio resources on the same carrier frequencies by coordinating transmissions across macro cells/RBSs and under laid cells/RBSs. In inter-cell interference coordination (ICIC), certain radio resources are allocated for the macro cells during some time period, and the remaining resources can be accessed by the under-laid cells without interference from the macro cell. Depending on the traffic situations across the layers, this resource split can change over time to accommodate different traffic demands. In contrast to the above split of carrier frequencies, this way of sharing radio resources across layers can be made more or less dynamic depending on the implementation of the interface between the nodes or points, e.g. the RBSs. In LTE, an X2 interface is specified that allows exchange of different types of information between radio base station nodes. One example of such information exchange is that an RBS can inform other RBSs that it will reduce its transmit power on certain resources.
Time synchronization between RBS nodes is required to ensure that ICIC across layers will work efficiently in heterogeneous networks. This is important for time domain-based ICIC schemes where resources are shared in time on the same carrier.
LTE uses Orthogonal Frequency-Division Multiplexing, OFDM, in the downlink and Discrete Fourier Transform, DFT,-spread OFDM in the uplink. The basic LTE physical communication resources can thus be seen as a time-frequency grid, as illustrated in the example in FIG. 2, where each resource element corresponds to one subcarrier during one OFDM symbol interval (on a particular antenna port).
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame including ten equally-sized subframes of 1 ms as illustrated in FIG. 3. A subframe is divided into two slots, each of 0.5 ms time duration.
The resource allocation in LTE is described in terms of resource blocks, where a resource block corresponds to one slot in the time domain and 12 contiguous 15 kHz subcarriers in the frequency domain. Two consecutive resource blocks (in time) represent a resource block pair and correspond to the time interval upon which transmission scheduling operates
Transmissions in LTE are dynamically scheduled in each subframe, where the RBS transmits downlink assignments/uplink transmission grants to certain UEs (mobile radio stations in LTE) via the physical downlink control channel, PDCCH. The PDCCH signals are transmitted in the first OFDM symbol(s) in each subframe and span (more or less) the whole system bandwidth. A UE that has decoded a downlink assignment carried by a PDCCH knows which resource elements in the subframe that contain data aimed for the UE. Similarly, upon receiving an uplink transmission grant, the UE knows which time/frequency resources it should transmit upon. In LTE downlink, data is carried by the physical downlink shared channel, PDSCH, and in the uplink, the corresponding data channel is referred to as the physical uplink shared channel, PUSCH.
Demodulation of transmitted data requires estimation of the radio channel which is done by using transmitted reference symbols, RSs, i.e. symbols already known by the receiver. In LTE, cell-specific reference symbols, CRSs are transmitted in all downlink subframes, and in addition to assisting downlink channel estimation, they are also used for mobility measurements and for uplink power control performed by the UEs. LTE also supports UE-specific RS aimed only for assisting channel estimation for demodulation purposes.
FIG. 4 illustrates a mapping of physical control/data channels and signals onto resource elements within a downlink subframe. In this example, the PDCCHs occupy the first out of three possible OFDM symbols, so in this particular case the mapping of data could start at the second OFDM symbol. Since the CRS is common to all UEs in the cell, the transmission of CRS cannot be easily adapted to suit the needs of a particular UE. This is in contrast to UE-specific RS where each UE has RS of its own placed in the data region of FIG. 4 as part of the PDSCH.
The length of the control region, which can vary on subframe basis, is conveyed in the Physical Control Format Indicator Channel, PCFICH. The PCFICH is transmitted within the control region at locations known by UEs. After a UE decodes the PCFICH, it knows the size of the control region and in which OFDM symbol the data transmission starts.
Also transmitted in the control region is the Physical Hybrid-ARQ Indicator Channel. This channel carries ACK/NACK responses to a UE to inform if the uplink data transmission in a previous subframe was successfully decoded by the base station or not.
Before an LTE terminal, e.g. a UE, can communicate with an LTE network it first has to find and acquire synchronization to a cell or RBS within the network, i.e., performing cell search. Then it has to receive and decode system information needed to communicate with and operate properly within the cell or RBS, and finally access the cell by a random-access procedure.
In order to support mobility, a UE needs to continuously search for, synchronize to, and estimate the reception quality of both its serving cell/RBS and neighbour cells/RBSs. The reception quality of the neighbour cells, in relation to the reception quality of the current cell, is then evaluated in order to conclude if a handover (for UEs in connected mode) or cell re-selection (for UEs in idle mode) should be carried out. For UEs in connected mode, the handover decision is taken by the network based on measurement reports provided by the UEs. Examples of such reports are reference signal received power (RSRP) and reference signal received quality (RSRQ). Depending on how these measurements, possibly complemented by a configurable offset, are used, the UE can for example be connected to the cell/RBS with the strongest received power, the cell/RBS with the best path gain, or something between the two.
These selection strategies do not result in the same selected cell as the RBS output powers of cells of different type differ. This is sometimes referred to as link imbalance. For example, looking at FIG. 5, the output power of a pico RBS 510 or a relay is in the order of 30 dBm or less, while a macro RBS 500 can have an output power of 46 dBm. Consequently, even in the proximity of the pico cell, the downlink signal strength from the macro RBS 500 can be larger than that of the pico RBS 510. From a downlink perspective, it is often better to select cell/RBS based on downlink received power, whereas from an uplink perspective, it would be better to select cell/RBS based on the path loss.
In the above scenario, it might be better, from a system perspective, to connect to the pico cell/RBS 510 even if the macro downlink is much stronger than the pico cell downlink. However, ICIC across layers would be needed when UEs operate within the region of the UL border 511 and the DL border 512. This area is also referred to as the link imbalance zone. Some form of interference coordination across the cell layers is especially important for the downlink control signalling. If this interference situation is not handled appropriately, a UE in the region between the DL and UL borders in FIG. 5 and connected to the pico RBS 510 cannot receive the downlink control signalling from the pico RBS 510.
One approach for providing ICIC across layers is illustrated in FIG. 6, where an interfering macro RBS (downlink interference towards a pico cell/RBS) avoids scheduling unicast traffic in certain subframes implying that neither PDCCHs nor PDSCH occur in those subframes. In such way, it is possible to create low interference subframes, which can be used to protect pico users operating in the link imbalance zone, a pico user being a UE connected to the pico RBS. The macro RBS (MeNB) indicates via the backhaul interface X2 to the pico RBS (PeNB) which subframes it will avoid scheduling UEs within. The PeNB can then take this information into account when scheduling UEs operating within the link imbalance zone; such that these UEs are scheduled in subframes aligned with the low interference subframes at the macro layer, i.e. in interference protected subframes. However, pico cell UEs operating within the DL border can be scheduled in all subframes, i.e. in both protected and non-protected subframes.
In principle, data transmission in different layers could also be separated in the frequency domain by ensuring that scheduling decisions in the two cell layers are non-overlapping in the frequency domain, e.g., by exchanging coordination messages between the different RBSs. For control signalling, this is not possible according to the LTE specifications where control signalling spans the full bandwidth, and hence, a time-domain approach must be used.
One way to deploy a network is to let different transmission/reception points form separate cells. In other words, signals transmitted from or received at a point are associated with a cell-id that is different from the cell-id employed for other nearby points. Typically, each point transmits its own unique signals for broadcast (e.g., Physical Broadcast Channel, PBCH) and synchronisation channels (e.g., Primary Synchronisation Signal, PSS, and Secondary Synchronisation Signal SSS).
The concept of a point is often used in conjunction with techniques for coordinated multipoint, CoMP. In this context, a point corresponds to a set of antennas covering essentially the same geographical area in a similar manner. Thus a point might correspond to one of the sectors at a site, but it may also correspond to a site having one or more antennas all intending to cover a similar geographical area. Often, different points represent different sites. Antennas correspond to different points when they are sufficiently geographically separated and/or having antenna diagrams pointing in sufficiently different directions. CoMP techniques introduce dependencies in the scheduling or transmission/reception among different points, in contrast to conventional cellular systems where a point from a scheduling point of view is operated more or less independently from the other points.
This typical strategy of one cell-id per point is depicted in FIG. 7 for a heterogeneous deployment where a number of low power (pico) points or RBS are placed within the coverage area of a higher power macro point or RBS. Similar principles also apply to classical macro-cellular deployments where all points have similar output power and perhaps placed in a more regular fashion than what may be the case for a heterogeneous deployment. In FIG. 7, one macro point or macro RBS 700 is illustrated having a coverage area or cell 701. The cell 701 has cell-id 1. Within macro cell 701, three different low power points/RBSs 710, 720 and 730 are deployed. Each low power point/RBS has a coverage area or pico cell 711, 721 and 731 respectively. The three different pico cells have their own specific cell-id, i.e. pico cell 711 has cell-id 2, pico cell 721 has cell-id 3 and pico cell 731 has cell-id 4.
An alternative to the typical deployment strategy is to instead let all the UEs within the geographical area outlined by the coverage of the high power macro point be served with signals associated with the same cell-id. In other words, from a UE perspective, the received signals appear coming from a single cell. Looking at FIG. 7, all cells 701, 711, 721 and 731 have the same cell-id, e.g. cell-id 1. Only one macro point/RBS 700 is shown, and other macro points would use different cell-ids (corresponding to different cells) unless they are co-located at the same site (corresponding to other sectors of the macro site). In the latter case of several co-located macro points, the same cell-id may be shared across the co-located macro-points and those pico points that correspond to the union of the coverage areas of the macro points. Synchronisation, Broadcast Channel, BCH, and control channels are all transmitted from the high power point while data can be transmitted to a UE also from low power points by using shared data transmissions PDSCH relying on UE specific RS. Such an approach has benefits for those UEs capable of PDSCH based on UE-specific RS while UEs only supporting CRS for PDSCH (which is likely to at least include all LTE Release 8/9 UEs for Frequency Division Duplex, FDD) must settle with the transmission from the high power point and thus will not benefit in the downlink from the deployment of extra low power points.
The single cell-id approach is geared towards situations in which there is fast backhaul communication between the points associated to the same cell. An example case might be an RBS serving one or more sectors on a macro level as well as having fast fibre connections to remote radio units (RRUs) playing the role of the other points sharing the same cell-id. Those RRUs could represent low power points with one or more antennas each. Another example is when all the points have a similar power class with no single point having more significance in than the others. The RBS handles the signals from all RRUs in a similar manner.
An advantage of the shared cell approach compared with the typical approach is that the typical handover procedure between cells/RBSs only needs to be invoked on a macro basis. Another advantage is that interference from CRS are greatly reduced since CRS do not have to be transmitted from every point. There is also greater flexibility in coordination and scheduling among the points sot the network can avoid relying on the inflexible concept of semi-statically configured “low interference” subframes, as illustrated in FIG. 6. A shared cell approach also allows decoupling of the downlink with the uplink so that for example path loss based reception point selection can be performed in uplink while not creating a severe interference problem for the downlink, where the UE may be served by a transmission point different from the point used in the uplink reception. Typically, this means that the UE's uplink transmissions are received by a pico point, while the UE receives downlink transmissions from the macro point.
According to 3GPP Release-10 LTE, uplink power control (UL PC) is performed by estimating a path loss (PL) term and combining it with various UE-specific and cell-specific power offset terms. An example power control (PC) formula from Rel-10 is in the formP=min(Pmax,10 log 10(M+P0+α*PL+C))[dBm]  (1)where Pmax represents a cap on the output power (in dBm), M represents the scheduled UL bandwidth, P0 is a UE- and/or cell-specific power offset, α is a cell-specific fractional path loss compensation factor, PL is an estimate of the path loss performed by the UE and C is a correction term possibly obtained as a combination of multiple power correction terms, possibly including closed-loop power control correction terms.
The UE estimates the path loss PL based on the difference (in dB) between the received power for cell-specific common reference signals (CRS) and the nominal power of such reference signalsPL=referenceSignalPower−higher layer filtered RSRP  (2)where referenceSignalPower is configured by higher layer signalling and RSRP is defined for the reference serving cell. Filtering of the RSRP may be configured by higher layer signalling and performed by the UE. The serving cell chosen as the reference serving cell and used for determining referenceSignalPower and higher layer filtered RSRP is configured by the higher layer parameter pathlossReferenceLinking.
A problem with uplink power control is that decoupling the downlink operations from the uplink operations does not apply to the UE's open loop part of the output power setting because the UE regulates its transmit power based on the CRS's and a reference power level transmitted by the RBS. In some cases, the open loop part of the power control may completely determine the output power, e.g., when the UE is only using open-loop power control. In case the UE is served by a macro RBS/cell in the downlink, the RSRP measurement that determines transmit power will not take the pico nodes/RBSs into account, which means that the UE will transmit with a power level that causes the received power in the pico node/RBS to be far above what is determined by the UE-specific and/or cell-specific power offset P0. The network may then employ closed loop power control to steer the UE's output power to a value that it sees fit. This can be done by sending transmit power commands, TPCs, in uplink grants to the UE. The TPC is a two-bit instruction and can be either an absolute setting or an accumulative value. The accumulative value, which would be required to control the power over a large dynamic range, takes one of the four values [−1, 0, 1, 2] dB.
Due to the uneven output powers of the different nodes/RBSs and since the CRS are shared between macro and pico nodes/RBSs in a shared cell-ID setting, the UE power control will be detrimental towards achieving area splitting gains. Because the macro node/RBS has a much higher output power than pico nodes/RBSs, UEs that could be served by a pico RBS in the uplink will too often regulate their transmit power towards the macro RBS even though the macro RBS/cell has a much lower path gain that the pico RBS/cell. This power output will likely create excessive interference within the cell and thereby degrade the possibility of multi-user access (e.g. SDMA) within the cell. Also, the UE power consumption will be unnecessarily high if a too high output power is used.